Only players drafted after playing in the CHL must remain in the CHL or NHL until they are 20.
Players drafted out of Europe or the USHL or other leagues are eligible for the AHL at age 18, even if a CHL team has drafted them in its import draft.
I don't think Halttunen is going to the AHL (seems way too early, given his performance in the Liiga), so this could just be a way to get him under contract early or maybe they want to send him to London and signing him to his ELC was the way to get him to agree (he'll probably make more money off his signing bonus and junior salary than he would in Finland).